Most craters posted to the LROC Featured Image page are impact craters, however, Hyginus Crater (located at 7.75°N, 6.27°E, in Sinus Medii) is a volcanic crater known as a caldera. Two main pieces of evidence suggest that Hyginus Crater formed through volcanic processes. First, Hyginus lacks a raised rim typical of impact craters. Second, the rim of Hyginus is irregular (not circular), which is typical for volcanic craters caused by collapse. Also, if you look closely with the LROC NAC, the interior of Hyginus has a number of small irregular depressions which are most likely collapse features, indicating a volcanic origin for Hyginus. These irregular depressions are in the Featured Image, distinguished by rough, high reflectance material around their edges.

Eight meter per pixel resolution view from LROC QuickMap shows the "meniscus hollows" features in context with the eastern interior of Hyginus [NASA/GSFC/Arizona State University].

The entire Hyginus region, shown in the LROC context image below, is a complex piece of lunar real estate. Not only do you have the volcanic crater Hyginus, but also Rima Hyginus, a linear rille, more volcanic collapse craters aligned with the linear rille, and a pyroclastic deposit around the crater Hyginus. How do all the geologic features relate to one another? The Hyginus region is so amazing that it was a candidate landing site for the canceled Apollo 19 mission. Had events turned out differently, we might know much more about the pyroclastic materials and the Hyginus caldera. Continue reading below for a summary of the scientific theory of how the Hyginus region formed.

An almost oblique view (spacecraft and camera slewed 19.57° east from nadir), LROC WAC view from 42.29 kilometers over an area west of Rima Hyginus. The caldera, particularly its east walls, can be seen here in some relief, without the high angle of incidence seen in the next image. LROC WAC observation M165814883C (604nm), LRO orbit 9570, July 20, 2011; native resolution 62.94 meters [NASA/GSFC/Arizona State University].

In a recent paper, scientists proposed a model of formation for Hyginus crater and Rima Hyginus. First, a body of magma from the mantle rose vertically through the lunar crust. The magma stopped rising near the surface and spread out laterally. This introduction of new material beneath the surface caused stress on the crust, which resulted in faulting. Eventually, gasses from the magmatic material still underneath the surface built up and increased the gas pressure, further increasing the stress on the crust.

LROC Wide Angle Camera (WAC) monochrome (689nm) observation of the
Hyginus region. The yellow arrow marks
the location of the collapse feature, the meniscus hollow, within Hyginus caldera seen at
high resolution in the LROC Featured Image and the white arrow designates a small dome feature brought to attention by Maurice Collins. LROC
WAC M117447052ME, orbit 2442, January 6, 2010; incidence angle 81.75° and a 62.6 meter resolution from 41.63 kilometers
[NASA/GSFC/Arizona State University].

This stress eventually caused graben to open along the faults, and the same release of stress initiated an eruption, including pyroclastic materials. After the eruption of magmatic material an empty cavity beneath the surface was left behind. This cavity collapsed, creating Hyginus crater. The collapse craters along the linear rille also formed in a similar way.

LROC NAC image M126887222L gives another look at the largest collapse
feature in the main image. This image field of view is 487 meters wide (588 meters in the LROC Featured Image release), and has a lower illumination
incidence angle, which emphasizes albedo differences over the kind of relief visible in the LROC WAC image immediately above. LRO orbit 3833, April 26, 2010; incidence angle 28.28° with a resolution of 0.48 meters from 40.55 kilometers [NASA/GSFC/Arizona
State University].

The central and western Hyginus and Rima Hyginus region and points immediately north and beyond under mid-morning illumination, as seen from around 100 kilometers over the south , a forward-looking HDTV still captured by Japan's lunar orbiter SELENE-1 (Kaguya) in 2008 [JAXA/NHK/SELENE].